If you don't pull back hard on the stalk, it will spray the windshield but not the headlamps -- regardless of whether the lights are on, whether it is the third time you've done it, or whatever. At least, mine works that way and I doubt it's different for the 07 or 08.

We had another thread about this where some people disagreed with the above statement, but I've gone back and verified that it DOES work this way on my car. No need to disable the headlamp washers if you don't like them.